Demonstration video from Paper Mario: The Thousand-Year Door. The entire Pit of 100 Trials is completed when Mario is only at level 4 at the start, just after completing chapter 4. Levels 98-100 are snipped out into a separate vid in two sizes if you don't want the entire thing.
My goal was to complete the entire Pit of 100 trials at the lowest entry-level & stats possible, giving every level up to BP to remain at 10/5. I am at level 4 after chapter 4, which requires using fright masks on A LOT of battles to get only 1 star point from them, and of course avoiding unnecessary battles. I bring a select set of badges that I felt were necessary to survive Bonetail. The only really optional badge I brought was peekaboo, to see the enemy HP without tattles. Knowing how much HP they have is pretty important in the lower levels. I have yoshi leveled up so he does five attacks instead of four, and Koops leveled up to make the upper levels go faster. I used Goombella and Flurrie once each I think, and never Vivian.
The reason I wait until after chapter 4, is I think it's humanly impossible to beat the pit before then, when you get art attack. With save states you could do it with just earth tremor (after chapter 1) but you'd have to super guard hundreds of attacks... I can't do that.
Art attack of course uses 4 star power, so having it to do at the start of every battle would require a lot of appealing and running. Well, if you do the "can you find me" trouble, you get the first parlor card and you can buy more super appeal badges for 34 piantas. If you have 12 of them, a single appeal brings back about 3.4 star power. Throw in some stylish first strikes, and an appeal by your partner, and you get all 4 back... so now it's just one appeal and run. I should note that I use the classic 'shirt stretching' method for pressing A fast, and in this run I never failed a single run, which would usually be deadly.
An ideal art attack only does 15-18 damage to all enemies, and some of them have more, namely the d.koopatrol and the swampires. The d.koopatrols aren't a problem if I have spike shield, which I also use for spiked parabuzzies. For the swampires, since them attacking is BAD, I bring along thunder rages and buy more from Charlieton if I can. I like to call Charlieton "con-artist guy" because of his pricing scheme.
Then there's bonetail. I layed out a plan to massacre him with the use of power/mega rush [p] badges. After chapter 4 you can have one of each, unless you've gotten more from enemy drops, but I didn't want to go around trying for that. After chapter 5 you can get more power rushes from the casino, but I wanted to do this right after chapter 4. The plan required 2 boo's sheets to make me and yoshi invisible, and there's only two you can have at this point (again, ignoring item drops or audience members). One from the twilight town shop storage, and one from con-artist guy. Then I just needed a way to have me and yoshi at 1 hp for the fight. For yoshi it was easy... he's at 1 HP when I enter the pit, and I never use him until bringing him in at the end of 99. Level ups only max your current partner, so he stays at 1. Mario was a different story. The only way to get to 1 HP is to take damage, which is too risky; use trial stew, which also drops you to 0 FP; or use point swap when you have 1 FP. I decided point swap was the best, since I could double dip to go from 5->1 FP and then the point swap to give mario 1 HP and restore the FP. If I do that at the end of level 99 and then thunder rage to finish 99, i'm set right?
Wrong. I forgot all about the possibility of a level up on 99. That ruins the point swap and brings mario back to 10 HP. I realized this when I first made it down to 99 and got that level up, and proceeded to get destroyed by bonetail because of my lack of a plan. So, I layed one out in case it happened again.
I tried and tried... and kept screwing up. I probably took 12 hours of attempts in a week before I succeeded. One time I died on level 99 due to a huge brain fart. Instead of leaving one enemy alive, I left all 5. Another time I arrived at bonetail with no level up on 99, and had the battle ruined due to FOG. The next time I made it to him, it was with a level up on 99, but I was disoriented from having thrown away the point swap already and managed to forget to change my badge layout... so I could barely hurt him at all and was quickly dead. The next time, I finally finished.
I don't want to start the appeal->run chain until after level 50, because that's when the enemies get a lot harder. So, before then I just use normal fighting... although I do use art attack a few times, namely on the hyper clefts, because they take too long to take out otherwise. I never use earth tremor or sweet treat in the entire run. I brought along 2 life shrooms as insurance, and one of them was used on level 48 because of some failed blocks. Other than the boo's sheets and the point swap, the rest is one shooting star (from con-artist guy) and four thunder rages. After picking up the item sack on level 50 I buy more thunder rages if I see him. I also got two free ones, but it turns out I didn't need any of them, I only used four due to the way the fights turned out. What was more important was a free boo's sheet; I don't believe I'd have beat bonetail if I hadn't got it.
I ended up with a level up on 99 again, but I was prepared for it now... right? Wrong. My strat had a flaw, which I didn't realize until I was about to use the point swap on mario. Because of the flower saver p, I still had 2 FP left, so mario would get 2 HP which would only increase his attacks by 2. I bailed and just did a normal attack, then when the boo's sheets ran out, used the free one on yoshi so he wouldn't die. One power bounce from mario did a lot less damage than it would have if I had the +7 from the badges, but yoshi made up for it. Bonetail attacked again, and I got lucky. The attack would have killed me (unless I had guarded it), but since I still had the life shroom it actually didn't matter. One more yoshi attack kills him, even though the power lift has wore off.
I end up level 23 at the end, though in one attempt I was level 25 due to amazy dayzees giving tons of experience. I never saw any in this trip, and in fact I got a mover on level 95 which would have taken me straight to bonetail. I ignored all movers I saw though because I wanted to show the whole endurance thing of doing all the fights. If anyone thinks that they can do this without art attack... I'd really love to see it.